A nuclear spin tomograph in which a human body is positioned within an electromagnetic system which may comprise ferromagnetic parts, has been known already from German Disclosure Document No. 28 54 774. According to this arrangement, however, the patient is surrounded on all sides, at least during the measurement, by magnetic coils and electromagnetic bodies of an unspecified type, so that it is not apparent how the patient is to be introduced into the test space and how exactly the coils for generating the magnetic field are to be arranged. This makes the magnetic system of the known arrangement absolutely unsuited for practical use because there is no opening through which the test object can be introduced and because in addition it requires magnet dimensions which, for reasons of weight, rule out their realization as iron magnets.
The term nuclear spin tomography or NMR tomography is understood to describe a method in which a test object, in particular a live human body or animal body is subjected simultaneously to a strong homogenous magnetic field and an r.f. field directed perpendicularly thereto. At the same time, one generates in the area of the test object a gradient field acting in the same direction as the homogenous magnetic field and decreasing in strength in the axis of the test object. Now, when the r.f. field is applied, nuclear magnetic resonance occurs only in the area of one plane of the test object because, due to the active gradient field, the magnetic field has a strength which, given the gyromagnetic ratio of protons, corresponds to the frequency of the irradiated r.f. field only in the area of this same plane. This makes it possible to produce sectional images of the test object.
In order to obtain sufficient measuring signals, it is necessary to work at relatively high magnetic field strengths. Known nuclear spin tomographs operate, for example, at a magnetic field having a strength of 0.23 Tesla.
On the other hand, relatively large sample spaces are required to permit, for example, measurement on human bodies. This is the reason why one has used to this day mainly magnetic systems using air-cored coils which were operated either in the normally conductive or in the superconductive operating mode. However, such air-cored coils have quite a number of drawbacks.
Firstly, the extreme homogeneity of the magnetic field required in the sample space is impaired already by minor disturbing factors occurring outside the magnetic system. Such disturbing factors may be either of a stationary nature, as for example reinforcing steel in the walls of the room in which the nuclear spin tomograph is installed, or else of a moving type, such as instrument trolleys moved in the neighborhood of the tomograph or even cars passing outside the examination building.
Secondly, air-cored coils give rise to a considerable leakage field since the homogenous magnetic field extending along the axis of the air-cored coil closes via the external space of the air-cored coil. This leakage field of the air-cored coil may influence equipment placed near the magnet system, such as electronic equipment, data processing systems or radiological equipment in a hospital. In addition, the leakage field may also disturb the operation of pacemakers so that it may be connected with certain risks for pacemaker patients to stay near the magnet system.
In an effort to overcome these disadvantages, a known system described in German Disclosure Document No. 31 23 493 has been provided with a shielding of a soft-magnetic material which absorbs the largest part of the leakage field. But due to the solenoid design of the air-cored coil, in which the direction of the constant magnetic field coincides with the coil axis, a considerable portion of the leakage field is still permitted to leak out through the opening which is required for introducing the test object.
In addition to these disadvantages relating to the leakage field, magnet systems using air-cored coil arrangements in the form of Helmholtz arrangements or of solenoids are connected with still another disadvantage resulting from the coil geometry.
As mentioned before, the constant magnetic field extends in the case of these coils along the direction of the coil axis and, thus, also the longitudinal axis of the patient. The coils required for irradiating the r.f. field therefore have to be of the saddle-shaped type as the r.f. field must extend perpendicularly to the constant magnetic field. However, these saddle-shaped r.f. coils, which serve not only for radiating the r.f. field, but also for receiving the measuring signal, have a relatively poor efficiency which in practice is lower by the factor 2.5 than the one that can be achieved with the aid of an r.f. transmitting and receiving coil in the form of a solenoid.
Finally, normally conductive air-cored systems offer the known disadvantage that the conversion ratio, i.e. the ratio between the achieved field strength and the consumed electric energy, is relatively poor so that large power supplies with a considerable energy consumption are required for feeding such normally conductive air-cored coil systems.
In assessing the value of the present invention, it should further be noted as an important aspect that the standard works in the field of nuclear spin tomography describes the use of electromagnets or measurements on human bodies as not feasible for reasons of dimension (e.g. Electro/78 Conference Record, May 23 to 25, 1978, pages 30/21-15, Boston, USA). This is affirmed also by the nuclear spin tomographs presently available on the market with use exclusively air-cored arrangements. Hence, it is the credit of the present invention that it was the first to overcome this prejudice and to indicate a way of implementing a nuclear spin tomograph for measurement on human bodies using an iron magnet.
Now, it is the object of the present invention to improve nuclear spin tomographs of the type described above so that any magnetic disturbing factors acting from the outside to the inside, or from the inside to the outside, are minimized; that r.f. coil arrangements of high efficiency can be used; and that relatively little electric power has to be employed to achieve high field strength, while maintaining on the other hand the full advantages of the known devices, in particular its ease of access to the test space.